1. Field of the Invention
The present invention relates to a light-controlled light modulator, and more particularly to a light-controlled modulation technique for modulating light with a wavelength identical to or different from input signal light with an arbitrary wavelength in response to the intensity of the input signal light in a wavelength division multiplexed optical network.
2. Description of the Related Art
Conventionally, as an optical transmission system for transmitting optical signals with different wavelengths, an optical transmission system using wavelength division multiplexing (WDM system) has been known which transmits the optical signals with different wavelengths by combining them into a single optical fiber. Recently, such WDM systems have been increasingly applied not only to one-to-one transmission, but also to network transmission.
In such WDM systems, a light-controlled light modulator is increasing its importance which carries out wavelength conversion, that is, which converts the wavelength of an optical signal traveling through an optical fiber into the same or different wavelengths.
FIG. 1 is a diagram showing a circuit of a conventional wavelength converter. The wavelength converter consists of a symmetric Mach-Zehnder interferometer that comprises SOAs (Semiconductor Optical Amplifiers) 105 and 106, MMI (Multi-Mode-Interference) couplers 101, 102 and 103 connected to the SOAs 105 and 106, an MMI coupler 104 connected to the MMI couplers 102 and 103, and optical waveguides interconnecting them. In FIG. 1, the reference numeral 107 designates signal light, 108 designates continuous light, 109 designates output light and 110 designates a port.
The operation of the wavelength converter with such a configuration will now be described.
The continuous light (CW light) 108 with a wavelength λj is launched into the MMI coupler 101, and split into two optical waveguides. The two continuous light waves pass through the SOAs 105 and 106 and the MMI couplers 102 and 103, and are coupled by the MMI coupler 104 to be emitted from the port 110.
In this state of the wavelength converter, the optical signal λi(s) 107 with the wavelength λi is launched into the MMI coupler 102, and then into the SOA 105. Here, the optical signal 107 varies the refractive index of the SOA 105.
As a result, the interference conditions change of the symmetric Mach-Zehnder interferometer comprising the MMI couplers 101, 102, 103 and 104 so that only when the signal light 107 is “1”, the output light with the wavelength λj is emitted from the port 109. Thus, the optical signal with the wavelength λi is transformed to light with the wavelength λj to be emitted from the port 109 as the output light λj(s).
In this method, the transmission rate of the input signal light is limited by the recovery time of carrier density changes of the SOAs 105 and 106. Thus, the speed of the wavelength conversion of the optical signal is limited to about 20 Gbps at most.
FIG. 2 is a diagram showing another conventional wavelength converter. The wavelength converter comprises an SOA 201, MMI couplers 202 and 203 connected to the SOA 201, and a loop-type interferometer 209 connected between the MMI couplers 202 and 203. In FIG. 2, the reference numeral 204 designates signal light, 205 designates continuous light, 206 designates counterclockwise traveling light, 207 designates clockwise traveling light, 208 designates output light and 210 and 211 each designate a port.
In the configuration, the continuous (CW) light 205 with the wavelength λj is launched into the MMI coupler 203 through the port 211, and is split into two parts by the MMI coupler 203, which are delivered to the loop-type interferometer 209. In the loop-type interferometer 209, the two parts travel around the loop as the clockwise traveling light 207 and counterclockwise traveling light 206, are recombined by the MMI coupler 203 to be emitted from the port 211.
In this state, the signal light λi(s) 204 with the wavelength λi is launched into the MMI coupler 202. The incident signal light 204 passes through the SOA 201, which varies its refractive index. Thus, the light with the wavelength λj traveling in the loop is affected by the change in the refractive index, resulting in phase variations as shown in FIG. 3A.
The clockwise traveling light 207 brings about abrupt phase variations, followed by recovering of the phase at a rate corresponding to the recovery time of carrier density changes of the SOA 201, and is launched into the MMI coupler 203.
The counterclockwise traveling light 206 also undergoes similar phase variations. However, since it travels through the loop-type interferometer 209 longer than clockwise traveling light 207 by a distance ΔL, it is launched into the MMI coupler 203 with a delay time Δτ.
Accordingly, in the MMI coupler 203, the time of the phase variations differs by an amount of Δτ=ΔL/(c/neq0) between the clockwise traveling light 207 and the counterclockwise traveling light 206, where c is the speed of light, and neq0 is the equivalent refractive index of the waveguide constituting the loop. The two continuous light waves with the same wavelength λj interfere with each other in the MMI coupler 203. In the course of this, their phases differ only during the time period Δτ and nearly equal thereafter. As a result of the interference, the light is emitted from the port 210 only during the time slot Δτ as illustrated in FIG. 3B. In other words, the input optical signal with the wavelength λi is transformed to the light with the wavelength λj to be output to the port 210 as output light λj(s) 208.
In the wavelength converter with such a loop-type interferometer, the counterclockwise traveling light 206 and the clockwise traveling light 207 have the same phase variations during the time the light phase variations gradually recover in response to the carrier concentration in the SOA 201, except for the time period Δτ. Therefore, as a result of the interference, the effect of the variations in the refractive index in the SOA 201 are canceled out, and the light with the wavelength λj is emitted from the port 211 except for the time period Δτ. Thus, as illustrated in FIG. 3B, the waveform after the wavelength conversion output from the port 210 includes no low-rate components whose rate is limited by the recovery time due to the carrier density changes, enabling high-speed wavelength conversion with steep rising and falling edges.
In the wavelength converter with the loop-type interferometer 209, however, the input signal light 204 is combined and output from the same port 210 as the output light 208. Therefore, to separate the output light 208 from the input light 204, a wavelength filter 212 must be connected to the output port 210 to extract only the output light 208.
Furthermore, when the wavelength λi of the signal light is the same as the wavelength λj of the wavelength conversed light, the wavelength filter 212 cannot separate them. This means that the light before the wavelength conversion is mixed into the output light as noise. Thus, it has a problem of being unable to carry out the conversion of the same wavelength. In addition, since the 3 dB coupler 202 is used to split the continuous light 205 with the wavelength λj, it has a problem of bringing about 3 dB additional loss in principle.
Moreover, when the wavelength converter with the loop-type interferometer is used, it is necessary that the length of the SOA 201 is sufficiently smaller than ΔL as illustrated in FIG. 2. More specifically, since the clockwise traveling continuous light 207 travels in the same direction as the signal light 204, it undergoes the effect of the variations in the refractive index throughout the length LSOA of the SOA 201. In contrast, since the counterclockwise traveling continuous light 206 travels in the direction opposite to the signal light 204, it does not undergo any effect of the variations in the refractive index until it encounters the signal light 204. Besides, since the effect of the variations in the refractive index varies depending on the position they encounter, the phase variation requires a rising time tr=2×LSOA/(c/neq), where c is the speed of light and neq is the equivalent refractive index of the SOA.
As a result, if the length of the SOA 201 is in the same order as ΔL, the phase variations of the clockwise and counterclockwise traveling continuous light waves become as illustrated in FIG. 4A, thereby deforming the waveform of the converted light emitted from the port 210 as illustrated in FIG. 4B because of the interference. Thus, it presents a problem in that the converted light emitted from the port 210 reduces its intensity as compared with that of FIG. 3B, thereby increasing the loss.